An optical data communication system can comprise one or more opto-electronic (light) sources that can be modulated at the data transmission rate, an optical transmission medium such as optical fiber, and one or more opto-electronic receivers that can demodulate the optical signals to recover the data. For high-speed optical data communication, the opto-electronic sources are commonly semiconductor lasers, such as vertical cavity surface-emitting lasers. Such lasers are preferred over other light sources, such as light-emitting diodes, because lasers are capable of operating at high power to achieve high bit rates. The optical fibers in high-speed, long-distance optical data communication systems are typically made of glass rather than an alternative optically transparent material such as plastic, because glass optical fiber exhibits low signal attenuation and low mode dispersion. A glass optical fiber typically has a core diameter between about 10 and 200 micrometers (μm). A plastic optical fiber typically has a core diameter between 500 μm and 1 millimeter (mm). Accordingly, although plastic optical fiber is economical and easy to install, glass optical fiber is much more commonly used in optical data communication systems than plastic optical fiber.
Pulse amplitude modulation (PAM) is a form of signal modulation in which the communicated information is encoded in the amplitude of a series of signal pulses. For example, in the PAM nomenclature, PAM4 refers to a technique in which the communicated information is processed in two-bit groups, where each group is mapped to one of four possible signal amplitude levels.